Repetitive exercise has been found to produce and increase the reserve capacity of the coronary vascular bed. This functional enlargement may furnish protection to a heart that is stressed more by the imposition of a pressure load by aortic constriction. An increase in pressure load will stimulate the heart to enlarge which can result in heart failure. The physiological, biochemical, and ultrastructural changes in repetitive exercise followed by pressure load by aortic constriction will be studied in the chronic instrumented dog. The dogs will be studied during a control period and subsequent to repetitive exercise with aortic constriction. Points of comparison will be based on the degree of heart enlargement during the course of the study. Physiological data will be obtained in the laboratory and during submaximal exercise tests. Laboratory measurements will include the coronary hyperemic response, cardiac pacing and response to various drugs. The biochemical function of the mitochondria, sarcoplasmic reticulum, lysosome, and myosin ATPase will be correlated with the physiological changes. Ultrastructural changes in the cardiac muscle will be determined by quantitative scanning and transmission electronmicroscopy. Any beneficial effect of repetitive exercise should change the relationship between cardiac enlargement and myocardium oxygen consumption. BIBLIOGRAPHIC REFERENCES: Sordahl, L.A., Asimakis, G.K., Dowell, R.T., and Stone, H.L.: Functions of selected biochemical systems from the exercised-trained dog heart. J. Appl. Physiol. 42: March, 1977. Asimakis, G.K. and Sordahl, L.A.: Effects of atractyloside and palmitoyl coenzyme A on calcium transport in cardiac mitochondria. Arch. Biochem. Biophys. 179: Feb., 1977.